Hi Pete @Pete_Violin - the only way I can sort of explain what I *believe* is happening is this - and I may not be describing this in the best technical manner (the actual detail is WAY outside of my knowledge base) - but I offer what is only my current best-understanding below...

The violin strings, when played, in themselves generate little sound.  The sound comes from the resonances both of the cavity and the plate structure of the violin (and other parts).   The violin has far from what could be described as a "linear frequency response" across the range of frequencies it could possibly play.  You could perhaps picture the violin's own acoustic spectral response as something like a hypothetical audio graphic-equalizer with thousands of narrow-band frequency settings adjusted by thousands of little sliders (!) across the audio spectrum of interest.   Rather than looking like a "band-pass filter" - or even a graphic equalizer say set to give bass, or treble lift, it will have lots and lots of peaks and troughs.  The peaks are providing "lift" at their specific frequencies, the troughs are (relative to the peaks) somewhat attenuating - or - at least - not lifting-as-much would be better to say.  It is not surprising then, that different notes/spot frequencies can just "sound different" to some extent, even from an adjacent note a semi-tone away.

My first observation of this was on my FiddlerMan Concert, in fact I wrote about it somewhere because it initially surprised me.  With the original strings that came with the instrument, my observation was quite clearly that around the C-C# the instrument's voice was louder and more resonant than adjacent notes, and had a "different quality" that's all.  However I did find two things - (1) this "effect" if you want to call it that, was to some extent string dependent - and when I tried a different string set some time later, it behaved quite differently, with a really difficult to describe sort of "dead" feel to the sound, where (and I appreciate a stopped string doesn't tend to ring-on much once the bow is lifted - certainly in comparison to an open string, but it does to some extent) and on that string, the sound just dampened really quickly (compared to adjacent notes).    (2) Sound post adjustment (pretty rough and ready I have to admit), and although small in movement modified the response.   My cheap old SkyLark (built like a barn door) did not exhibit this behaviour, but then again, it was largely dead from anything above the E on the A (which is why I transformed it into a pretend viola!).  My FM MJZ905 does appear to have this resonance effect, but to a much lesser extent than the Concert

You'll find some reference to what I'm rambling on about here - including a typical "acoustic efficiency" spectrum - which is what I meant by concept of the violin acoustics behaving like the "hypothetical graphic-equalizer" above.  Indeed, on that graph (on the link below) there is a dip in the curve somewhere around the c-c# area - the link is not https secure - but perfectly safe to visit - http://newt.phys.unsw.edu.au/j.....intro.html

Beats me, as they say -